Nonaqueous-electrolyte batteries including lithium secondary batteries are being put to practical use in extensive applications ranging from power sources for so-called public use, such as portable telephones and notebook type personal computers, to vehicle-mounted power sources for driving motor vehicles or the like. However, recent nonaqueous-electrolyte batteries are increasingly required to have higher performance and, in particular, there is a desire for improvements in various battery characteristics including high capacity, low-temperature use characteristics, high-temperature storability, cycling characteristics, and safety during overcharge.
The electrolytic solutions for use in nonaqueous-electrolyte batteries are usually constituted mainly of an electrolyte and a nonaqueous solvent. Used as main components of the nonaqueous solvent are cyclic carbonates such as ethylene carbonate and propylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, and cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone.
Various investigations have been made on nonaqueous solvents, electrolytes, and additives in order to improve the battery characteristics, such as load characteristics, cycling characteristics, and storability, of such nonaqueous-electrolyte batteries or to enhance the safety of the batteries during overcharge. Patent document 1 proposes a technique in which an additive that polymerizes at battery voltages higher than the maximum working voltage of batteries is incorporated into an electrolytic solution to thereby protect the battery by increasing the internal resistance thereof. Patent document 2 proposes a technique in which an additive that polymerizes at battery voltages higher than the maximum working voltage of batteries and that thereby evolves a gas to elevate the pressure is incorporated into an electrolytic solution to thereby enable an internal breaker disposed for protection from overcharge to work without fail. Disclosed as these additives are aromatic compounds such as biphenyl, thiophene, and furan.
Patent document 3 proposes a nonaqueous-electrolyte secondary battery system including both a nonaqueous-electrolyte secondary battery in which phenylcyclohexane has been added to the nonaqueous electrolytic solution in an amount of 0.1-20 parts by weight in order to inhibit the decrease in battery characteristics due to the use of biphenyl or thiophene and a charge control system which senses an increase in battery temperature to break the circuit for charge.
Patent documents 4 to 9 propose techniques in which various aromatics including cyclohexylbenzene are added to electrolytic solutions, and problems concerning improvement in safety during overcharge and durability have been solved to some degree.
Patent document 10 proposes a technique in which in order to enable a nonaqueous-electrolyte secondary battery to combine cycling characteristics and safety during overcharge, 2,2-diphenylpropane or the like is added to the electrolytic solution.
Patent document 11 proposes a technique in which in order to improve the cycling characteristics of a nonaqueous-electrolyte secondary battery, a benzyl alkyl carbonate or a 5-membered or 6-membered lactone containing a phenyl group is incorporated into the nonaqueous electrolytic solution.
Patent document 12 proposes a technique in which in order to reduce gas evolution in a nonaqueous-electrolyte secondary battery during high-temperature storage while maintaining the capacity, an aromatic ester compound or the like is incorporated into the nonaqueous electrolytic solution.
Examples of methods for increasing capacity which are being investigated include: a method in which the active-material layers of electrodes are pressed and densified to thereby minimize the volume within the battery which is occupied by components other than the active materials; and a method in which the range over which a positive electrode is utilized is widened to use the positive electrode up to a higher potential. However, in cases when the active-material layer of an electrode is pressed and densified, it becomes difficult to evenly use the active material and the reactions come to proceed unevenly, resulting in partial deposition of lithium and accelerated deterioration of the active material. It is hence difficult to obtain sufficient properties. Meanwhile, in cases when the range over which a positive electrode is utilized is widened to use the positive electrode up to a higher potential, the positive electrode has further enhanced activity and the deterioration thereof is prone to be accelerated by the reaction between the positive electrode and the electrolytic solution.
Patent document 13 proposes a technique in which in order to enable a nonaqueous-electrolyte secondary battery to combine cycling characteristics and high-temperature storability with safety during overcharge, the diacetate of bisphenol A or the like is incorporated into the nonaqueous electrolytic solution.